Lenses of the type contemplated by the present invention are generally characterized as ocular lenses and encompass lenses intended for direct contact with the eye (including both corneal and scleral type lenses). In addition, the term "contact lens" as employed herein is intended to include not only conventional contact lenses which are generally arranged on the surface of the eye but also intraocular or insert-type lenses commonly employed as surgical implants.
The term "contact lens" includes both scleral type lenses as well as more conventional lenses commonly referred to as contact lenses. In this regard, scleral type lenses generally have an outer annular portion of the lens adapted for contact with the eye. A pocket or recess is formed between the eye and a central portion of the lens and can be filled either with ophthalmological solution or tear solution fluid. The more conventional contact type lens referred to above, by contrast, is in generally uniform contact with the eye except for a thin film of tear fluid or the like. It is also to be understood that the term "contact lens" includes other varieties of lenses such as soft lenses, hard lenses, etc.
Hard ocular lenses, such as contact lenses, were initially made exclusively from glass. As interest and experience increased in polymer technology, glass was replaced by poly(methyl methacrylate) which then became the standard composition for such lenses because of its toughness, optical properties and physiological inactivity, as well as relative ease of manufacture (as least compared to prior art at the time), for example by turning on a suitable lathe.
Although hard contact lenses formed either from glass or poly(methyl methacrylate) could be fabricated in a full range of optical corrections, both materials were essentially impermeable to oxygen and therefore, as further explained below, could not be worn by a user for extended periods. Rather, the initial hard contact lenses were limited generally to daily usage. Although the hard contact lenses were also readily capable of sterilization, for example during overnight non-use, interest rapidly developed in contact lenses which could be worn for extended periods of time and which were inherently more comfortable.
This interest led to the development of so-called "soft contact lenses" which emerged with the development of a class of polymers generally referred to as "hydrogels". The key to the development of soft contact lenses was their relatively high water content yielding a soft flexible material with oxygen transport taking place through the body of the lens largely by means of the water component rather than the lens polymer itself. For this reason, so-called soft contact lenses were capable of extended wear, were immediately more comfortable and became highly popular.
However, soft contact lenses have tended to exhibit certain undesirable characteristics even though they usually present adequate oxygen permeability to avoid damage to the cornea over extended periods of wear. Such disadvantages include the inability to fabricate soft contact lenses to correct for all types of visual defects or to provide the full range of optical correction required for all users. Additionally, dehydration causes visual acuity to decrease during the days wearing period. Furthermore, soft contact lenses are generally characterized as being fragile and having a relatively short use life. Finally, soft contact lenses have been associated with infection of the eye from microorganisms and, therefore require a stringent sterilization and maintenance regimen.
The term "extended" use or wear may have various meanings in connection with contact lenses. Generally, although that term may refer to use or wear over a long term of, for example, thirty days, the term is used herein to signify use or wear at least overnight.
Even more recently, interest has developed in "rigid gas permeable" contact lenses or RGP contact lenses which combine the desirable features of hard contact lenses as noted above and the permeability and extended wear possibilities of soft contact lenses. Permeability is considered a fundamental requirement, particularly for RGP contact lenses, in order to permit the transport of atmospheric oxygen through the lens to the cornea. This is necessary because, unlike most tissues in the body, the human cornea lacks blood vessels for supplying oxygen to the cornea in the form of oxygenated blood. Rather, the cornea normally obtains oxygen directly from surrounding air. Contact lenses naturally interfere with this oxygen source, producing the need for gas permeability as noted above to avoid damage to the cornea, particularly during extended wear.
RGP contact lenses may be considered to have three essential requirements because of their function as an extension of the cornea. Initially, it is necessary as noted above to maintain a continuous undisturbed supply of oxygen to the cornea. As noted above, this is typically achieved by maintaining gas permeability in the contact lens itself. Secondly, it is necessary for the lens to be structurally stable at least to the extent of resisting deforming forces of the eyelid during blinking, for example in order to avoid visual instability. Thirdly, the composition of the lens must be such to provide surface wettability sufficient to enable a continuous tear film to be maintained between the lens and the cornea. At the same time, desirable surface characteristics include compatibility with the eye and the ability to avoid or minimize accumulation of proteinaceous material on the surface of the lens. Still other characteristics are also important, including comfort, coloration, and clarity. Finally, the material of an RGP ocular lens should be inexpensive to process into a completed lens. This characteristic is particularly important for low-cost lenses such as disposable lenses, which are becoming of greater interest.
The RGP contact lenses developed to date have been found satisfactory for certain of the above requirements with the possible exception of cost, comfort and, in some cases, wettability and structural stability. With available polymer technology, RGP lenses can incorporate relatively high oxygen permeability. At the same time, the RGP lenses can be fabricated in a broad range of optical corrections with the ability to correct most visual defects.
However, fabrication techniques for RGP contact lenses to date are relatively expensive, requiring techniques such as substantial machining on special lathes. It has further been found that contact lenses produced by these techniques tend to exhibit "creep", leading to changes in curvature of the lenses and compromising their structural stability. Additionally, some RGPs present comfort problems because of the lack of adequate wettability of some polymers and the inherent highly-rigid or non-flexible nature of the lens.
Generally, a broad range of polymers and combination of polymers and techniques have been considered to date in the development of desirable contact lenses. For example, a series of patents, as noted below, have disclosed a variety of linear co-polymers including acrylates for achieving desirable characteristics in RGP contact lenses.
Initially, Gaylord U.S. Pat. No. 3,808,179 issued Apr. 30, 1974 under assignment to Polycon Laboratories, Inc. disclosed contact lenses fabricated from a co-polymer of a fluoroalkyl acrylic ester and an alkyl acrylate or methacrylate to exhibit increased oxygen permeability. A wide variety of fluoroalkyl acrylic esters was disclosed in that patent.
Gaylord U.S. Pat. No. 4,120,570 issued Oct. 17, 1978 under assignment to Syntex (U.S.A.), Inc. disclosed yet another class of contact lens materials including in large part a polysiloxanylalkyl ester of a specified structure and allegedly having various improved functions such as improved oxygen permeability and surface wettability.
Gaylord Reissue Pat. No. 31,406 reissued Oct. 4, 1983 under assignment to Syntex (U.S.A.), Inc. further disclosed contact lenses fabricated from a co-polymer of a polysiloxanylalkyl acrylic ester (see the above patent) and an alkyl acrylic ester for the specified purpose of increased oxygen permeability. Another class of materials considered in contact lenses are silicone elastomers, the simplest of which may be characterized as poly(dimethylsiloxanes). A wide variety of such materials and reference to their possible use in contact lenses is noted in an article by Barry Arkles, "Look What You Can Make Out of Silicones", a reprint from CHEMTECH, 1983, 13, pp. 542-555 and Arkles U.S. Pat. Nos. 4,478,981 issued Oct. 5 23, 1984 and 4,550,139 issued Oct. 28, 1985.
Similarly, Laurin U.S. Pat. No. 3,994,988 issued Nov. 30, 1976 under assignment to Baxter Travenol Laboratories, Inc. disclosed co-polymers of polysiloxane, polycarbonate and polyester constituents particularly contemplated for a wide variety of medical applications including contact lenses.
Of related interest, a survey of various co-polymer systems was set forth in a book by Noshay and McGrath, Block Co-Polymers, Overview and Critical Survey, Academic Press, New York (1977), pp. 393, 394, et al. This reference is particularly noted in connection with the present invention in that it defines block co-polymers and sets forth numerous combinations of polymers which may be combined in block co-polymers useful for ocular lens compositions.
Additional block co-polymers particularly contemplated for use in contact or ocular lenses as defined by the present invention were disclosed in German Patent Application 2324654 filed May 19, 1973 under assignment to Biocontacts, Inc. from Stark, Auslander, Mandell and Marg. A corresponding disclosure appeared in French Application 2.185.653, Registration No. 73.181.11, also assigned to Biocontacts, Inc. from the same inventors. Both of these patents relied for priority on U.S. Pat. No. application Ser. No. 255,220, filed May 19, 1972 and subsequently abandoned. The above noted patents disclosed various block co-polymers of silicone and polycarbonate for forming contact lenses. Generally, the materials disclosed in these patents were not sufficiently stiff to permit machining.
Particularly in connection with the references noted immediately above, it is important to distinguish between block co-polymers and other co-polymers which are commonly referred to as linear or random co-polymers. Generally, as their name implies, block co-polymers are characterized by blocks or continuous chains of specific chemical species tending to demonstrate unique properties of the respective polymeric species.
By contrast, random co-polymers tend to be relatively short chain units, often with single monomer units in varying distribution along the chain link. In any event, the random co-polymers do not include clearly defined blocks of selected polymers as in block co-polymers.
Distinctions between block co-polymers and other co-polymers of the type referred to above are also set forth within a reference by Sperling noted and discussed in greater detail below.
Lim, et al, U.S. Pat. No. 4,536,554 issued Aug. 20, 1985 under assignment to Barnes-Hind, Inc. disclosed various compositions of hydrophilic polymers and contact lenses formed from those polymers, the Lim, et al. patent further disclosing transparent, optically clear interpenetrating network polymers for forming products such as contact lenses from such polymer systems. The interpenetrating polymer network (IPN) was specifically employed for combining two polymers in network form with one of the polymers being bound by the other polymer and allowed to swell to take on a substantial water content as high as 65% by weight. In any event, the IPN system of the Lin, et al. patent was specifically directed toward standard water-based soft hydrogel contact lenses.
The preceding references are believed to be fairly representative of the prior art. Furthermore, it is emphasized that although certain polymer systems have been developed lending themselves to specific applications in contact lenses, there remains a great need for a further improved contact lens, particularly a contact lens having a combination of many of the desirable properties; i.e., comfort, structural stability, high gas permeability, wettability, and clarity, while also having the ability to be manufactured in a simple, inexpensive manner, for example by molding, in order to particularly make the lenses available for relatively low-cost applications, such as for disposable use.